Carbon nanotubes are very small tube-shaped structures having the composition of a graphite sheet, formed as a tube. Carbon nanotubes produced by arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58).
Carbon nanotubes typically have excellent performance; in particular, good electron emission capability at low emission voltages, generally less than 100 volts. Furthermore, carbon nanotubes can carry high electric currents reliably. Due to these properties, carbon nanotubes are considered to be an ideal field emission material for application in a variety of field emission devices, such as planar light sources, field emission flat panel displays, etc.
A conventional nanotube-based planar light source typically includes a rear plate formed with a cathode, a face plate formed with an anode spaced apart from the cathode, and a vacuum cavity having a certain vacuum level defined between the rear plate and the face plate. The cathode usually includes a cathode conductive layer and a plurality of carbon nanotubes acting as an electron-emitting material formed on the cathode conductive layer. The anode usually includes a transparent anode conductive layer and a fluorescent layer formed on the anode conductive layer. However, the vacuum of the planar light source can degrade, during operation of the device, due to outgassing from the fluorescent layer. Outgassing primarily occurs when energetic electrons emitted from the electron-emitting material strike the fluorescent layer and cause trapped substance to be released. Absorption of the outgassed materials on the electron-emitting material may contaminate the electron-emitting material and result in large fluctuation in the emitted current of the electron-emitting material, even causes irreversible damage to the electron-emitting material. The outgassed materials may be atoms, molecules, atom clusters, ions, free radicals, etc. Common potential moleculars include, for example, O2, H2, N2, NH3, CO2, CO and H2O. Therefore, an adequate vacuum maintenance in the planar light source is a critical factor to provide a high stability emitted current.
What is needed is to provide a field emission cathode and a planar light source using the same, which can achieve a high stability emitted current.